What are the light blue lines on my imported objects
This means in Shading/UVs those edges have been marked as Sharp:

The Sharp mark is used by the split normals and the Edge Split modifier, which are part of the smoothing or customized shading techniques. As seams, it is a property of edges, and these operators set or unset it for selected ones.


See the manual: https://docs.blender.org/manual/en/latest/modeling/meshes/editing/edge/edge_data.html#mark-sharp-clear-sharp

As for clearing sharp edges, open up the Edge menu (CtrlE) and select Clear Sharp:

In 2.7 versions, this can be done here from the Tool Panel T &gt; Shading:

But this UI doesn't appear to be supremely intuitive (October 2015). it seems you have to click Sharp first before that panel becomes visible.

Here's a script that lets you

Mark selected edges as sharp
Or remove sharp status of a set of selected edges

Code for Object Mode:
import bpy

obj = bpy.context.object
for e in obj.data.edges:
    if e.select:
        # invert current state
        e.use_edge_sharp = (not e.use_edge_sharp)